In the art, it is desirable to produce elastomeric compounds exhibiting reduced hysteresis. Such elastomers, when compounded to form articles such as treads for tires, will show an increase in rebound, a decrease in rolling resistance and will have less heat build-up when mechanical stresses are applied. Nevertheless, modifying the tread compound has disadvantages, namely the sacrifice of other tire performance such as wear, wet traction, dry traction, snow traction and chipping and cracking of the tread.
The main source of hysteretic power loss has been established to be due to the section of the polymer chain from the last cross link of the vulcanizate to the end of the polymer chain. This free end cannot be involved in an efficient elastically recoverable process, and as a result, any energy transmitted to this section of the cured sample is lost as heat. It is known in the art that this type of mechanism can be reduced by preparing higher molecular weight polymers which will have fewer end groups. However, this procedure is not useful because processability of the rubber with compounding ingredients and during shaping operations decreases rapidly with increasing molecular weight.
Another method of reducing hysteresis has been to react a lithium end-capped elastomer with a tin chloride compound to give polymer chains with terminated tin. Tin has an affinity for carbon-black, which affinity reduces hysteresis by removing the effect of a free end. However, with a plurality of polymer chains, endcapping is an inefficient process that results in only about 50-80 percent of the total number of chains being capped with tin. While this method has provided a decrease in hysteresis, the large number of polymer chains without a tin endcap do nothing to reduce hysteresis.
It is also known in the art to employ tin-containing organo-metal compounds as polymerization initiators. For instance, U.S. Pat. No. 3,426,006 discloses the catalyst lithium tributyl tin in diethyl ether for such a purpose. This initiator has been shown by Tamborshi et al, Journal of Organic Chemistry, volume 28, page 237 (1963) to be predominantly an equilibrium mixture of dibutyltin and butyl lithium wherein the butyl lithium is the more active initiator and hence, the polymer chains produced from its initiation actually contain little or no tin atoms. Thus, heretofore, the art has not shown a means whereby substantially each polymer chain of an elastomer can be provided with a tin end group resulting from the initiator.
The present invention employs tire components other than the tread, containing or prepared from low hysteresis elastomers. In this manner, rolling resistance can be improved without sacrificing desirable properties of the tread. It is also possible to utilize tread compounds having lower hysteresis properties than conventional with these components for even greater performance. However, the hysteresis reduction in the tread does not need to be maximized at the expense of other tread properties because the lower hysteresis properties of the other components will improve the overall rolling resistance of the tire.
It is therefore, an object of the present invention to provide pneumatic tires having improved rolling resistance.
It is a further object of the present invention to provide a method of improving the rolling resistance of tires.
It is another object of the present invention to provide tire components such as subtreads, black sidewalls, body ply skims and bead fillers containing or prepared from elastomers having reduced hysteresis properties.
It is yet another object of the present invention to provide tire components prepared from elastomers having a plurality of polymer molecules wherein substantially each molecule has a functional group at the site of initiation.
It is still another object of the present invention to provide tire components such as subtreads, black sidewalls, body ply skims and bead fillers containing or prepared from elastomers having a plurality of polymer molecules wherein substantially each molecule has a functional group at the site of initiation to provide low hysteresis, which elastomers can be blended with other polymers to avoid significant loss of other physical properties such as tear strength.
It is still another object of the present invention to provide tire components such as subtreads, black sidewalls, body ply skims and bead fillers containing or prepared from elastomers having a plurality of polymer molecules wherein substantially each molecule has a functional group at the site of initiation to provide low hysteresis, which elastomers do not suffer significant loss of other physical properties such as tear strength as generally occurs when elastomers are chemically modified to reduce hysteresis.
It is still another object of the present invention to provide tire components prepared from elastomers having a plurality of polymer molecules wherein substantially each molecule has a functional group on the initiating end, and the molecules may be terminated with a tin-containing group or other functional group known to provide similar hysteresis reducing properties.
At least one or more of the foregoing objects together with the advantages thereof over the existing art, which shall become apparent from the specification which follows, are accomplished by the invention as hereinafter described and claimed.
In general, an improved pneumatic tire having decreased rolling resistance comprising at least one tire component selected from the group consisting of subtreads, black sidewalls, body ply skims and bead fillers, prepared from a vulcanizable elastomeric composition having reduced hysteresis comprising from about 10 to 100 parts by weight of an elastomer comprising a plurality of polymer molecules, wherein substantially each polymer molecule contains at least one functional group at the site of initiation, and having a hysteresis reduction of at least about 10 percent compared to the same elastomer without a functional group on substantially each polymer molecule; from about 0 to 90 parts by weight of an elastomer having greater hysteresis than the elastomer having reduced hysteresis and selected from the group consisting of natural rubber, synthetic rubber and blends thereof, and, from about 20 to about 100 parts by 5 weight of carbon black, per 100 parts of the elastomer.
A method of improving the rolling resistance of pneumatic tires comprises incorporating into the tire during manufacture at least one component selected from the group consisting of subtreads, black sidewalls, body ply skims and bead fillers, prepared from a vulcanizable elastomeric composition having reduced hysteresis comprising from about 10 to 100 parts by weight of a plurality of polymer molecules, wherein substantially each polymer molecule contains at least one functional group at the site of initiation, and having a hysteresis reduction of at least about 10 percent compared to the same elastomer without a functional group on substantially each polymer molecule; from about 0 to 90 parts by weight of an elastomer having greater hysteresis than the elastomer having reduced hysteresis and selected from the group consisting of natural rubber, synthetic rubber and blends thereof; and, from about 20 to about 100 parts by weight of carbon black, per 100 parts of the elastomer.